Zoom is used in digital cameras to crop out uninteresting parts of the view and to enlarge the interesting part of the view. Optical zoom genuinely enlarges the image and introduces new details and information to the image.
Digital zoom is also commonly included in digital cameras, either as a complement to the optical zoom or as the sole zooming method in low-end digital cameras. Digital zoom uses a primary image that is produced by camera optics and an imaging sensor as an input, and thus digital zoom does not introduce genuine new details to the image. Digital zoom either crops the input image and thus reduces the image resolution, or crops the input image and then interpolates, or in general, upscales the image to some higher resolution than the cropped resolution, for example, back to the original input resolution.
No information is lost from the ROI (Region of Interest) in the cropping but only the information outside the ROI. Similarly, no genuine new information is introduced in the interpolation; new values are estimated into each location (x, y) of the output image according to certain neighbourhood of pixels that correspond to location (x, y) in the cropped input image. For example, bilinear or bicubic interpolation are typically used in connection with this measure.
Typically the digital zoom in digital cameras operates in a following manner. The input primary image produced by the sensor and the optics is cropped according to the digital zoom factor and then the cropped image is interpolated back to the original resolution in order to produce the final output image. However, the activation of the digital zoom in the UI (User Interface) may vary.
Some manufacturers have a bit more sophisticated digital zoom methods. For example, Sony has a “SmartZoom” concept that is introduced in some devices [1]. The “SmartZoom” concept may operate in the following manner: 1) user selects output_resolution. This must be lower than the original_sensor_resolution. 2) The user enables SmartZoom. 3) The user uses zoom ring to zoom (i.e. optical zoom). 4) The system crops output_resolution_sized subimage from the original image. The resolution of that is original_sensor_resolution.
Thus, when using SmartZoom, the minimum zoom factor i.e. optical+digital is not ×1, but it is (original_sensor_resolution/output_resolution). This is bigger than ×1. The maximum zoom factor is increased to {optical_zoom_factor*(original_sensor_resolution/output_resolution)}.
SmartZoom does not deteriorate the image quality as much as the traditional digital zoom methods, which is due to use of simple cropping instead of interpolation+downscaling combination. However, the user cannot enable SmartZoom before he selects a lower resolution than original_sensor_resolution as the output_resolution.
Also, the HP PhotoSmart 935 digital camera [2] has at least the following digital zoom features 1) no interpolation is used, only cropping and 2) the actual pixel size that results from the digital zoom (cropping) is displayed on the viewfinder. According to quick tests, the JPEG compression factor seems to change from a fixed setting A into fixed setting B when a digital zoom operation is performed.
The aforementioned prior art solutions don't take into account further use applications. One of these further use applications is to produce a hard copy. Image quality problems arise due to the consecutive scaling measures that are performed in both directions (i.e. upscaling and downscaling). Problems may also arise due to the fact that scaling measures are performed as post-processing. This may cause the image, for example, to include artefacts.